1. Field of the Invention
This invention relates to a throttle opening control system for an internal combustion engine.
2. Description of the Related Art
In vehicle internal combustion engines, it is common to feedback-control the engine idling speed (conduct idling feedback control) when the throttle valve installed in the air intake system is at the fully-closed equivalent opening or angle (more precisely, a prescribed degree in the opening direction from the fully-closed opening or angle). This is achieved, for instance, by providing a bypass between the upstream and downstream sides of the throttle valve and regulating the amount of bypass air (secondary air) to be supplied to the engine by regulating the opening of an EACV (Electronic Air Control Valve) installed in the bypass when the throttle valve opening and other feedback control conditions such as the vehicle speed and the engine speed are satisfied, thereby controlling the actual idling speed to the desired idling speed.
Further, to cope with the fact that the mechanical fully-closed opening or angle of the throttle valve changes with aging, technologies have been developed for learning-controlling the fully-closed angle from the detected value of the throttle valve opening.
Specifically, the learned value of the fully-closed opening or angle in the learning-control is calculated from the deviation between a stored learned value and the throttle opening detected when the idling feedback control is executed (i.e., when the throttle valve is at the fully-closed equivalent opening or angle) and the calculated value is newly learned and stored (updated). However, vehicle operators sometimes lightly resting their foot on the accelerator pedal even when the engine is idling (thus causing the accelerator pedal to stay in a slightly depressed condition). When the throttle valve is slightly opened by such “pedal riding” during idling feedback control, the learned value is updated to a false open side value.
The throttle intake air amount (amount of air sucked in through the throttle valve) at the false fully-closed opening or angle learned and updated toward the opening direction is greater than that at the fully-closed opening or angle before the update. The idling feedback control therefore decreases the amount of bypass air by the amount of the increase in the throttle intake amount. If the operator discontinues pedal riding under these circumstances, the resulting decrease in the amount of air intake through the throttle valve temporarily or briefly reduces the total amount of air supplied to the engine to cause an undesirable drop in the engine speed.
In addition, the fully-closed opening or angle, which is one of the parameters used to determine whether to perform the idling feedback control, is reset based on the false fully-closed value that was learned, i.e., is reset to the opening calculated by adding the prescribed degree of opening mentioned above to the learned false fully-closed value. As a result, the throttle opening at which the determination to implement the feedback control is made is shifted in the opening direction. Therefore, if pedal riding is frequent and leads to a progressive increase in the throttle opening, the learned false fully-closed values will accumulate in the opening direction and cause the decline in the amount of bypass air to grow in proportion as the false learned values accumulate. When this situation arises, the total amount of intake air supplied to the engine decreases markedly at the moment pedal riding is discontinued, causing the engine speed to fall sharply and possibly leading to stalling of the engine.
For overcoming this problem, Japanese Laid-Open Patent Application No. Hei 9(1987)-53469, teaches a technique that checks for pedal riding and when pedal riding is discriminated, suspends idling feedback control in order to prevent decrease in the amount of bypass air.
This conventional technique cannot prevent the learning of false fully-closed value toward the opening direction during pedal riding because it learns (updates) the throttle valve fully-closed opening or angle regardless of whether the operator is riding the accelerator pedal. In other words, this prior art technique attempts to prevent engine speed destabilization during pedal riding by suspending engine speed control using learn-ed values. It is not capable of eliminating the various problems that arise when values learned during pedal riding are used for engine idling speed control. One undesirable effect of this conventional technique is that the suspension of idling feedback control during pedal riding makes it difficult to control the idling speed to the desired idling speed during pedal riding.